1. Field of the Invention
The present invention relates to a four-wheel steering system of a wheeled vehicle, more particularly to an electric control apparatus for a rear-wheel steering mechanism for controlling steerage of a set of dirigible rear road wheels in accordance with a yaw rate of the vehicle body.
2. Discussion of the Prior Art
In Japanese Patent Laid-open Publication No. 63-207772, there has been proposed an electric control apparatus of this kind which is designed to detect a yaw rate of the wheeled vehicle for determining a target steering amount for steering the rear road wheels in accordance with a magnitude of the detected yaw rate in a direction restraining the yaw rate of the vehicle and to produce a control signal indicative of the target steering amount for applying the control signal to an electrically operated actuator of the rear wheel steering mechanism. With such a conventional control apparatus, the yaw rate during turning of the vehicle converges as shown by two-dot chain lines in FIG. 5 of the present application to enhance the running stability of the vehicle. Under control of the conventional control apparatus, however, the rear road wheels are steered in response to the detected yaw rate of the vehicle body. As a result, the cornering force of the rear road wheels rapidly rises to shorten a transition period for turning of the vehicle, and as shown by the two-dot chain lines in FIG. 5, a rise rate of lateral acceleration of the vehicle body becomes steep, resulting in an increase of the roll angle speed of the vehicle body.